1. Field of the Invention
This invention relates to a process for the substantially full recovery of the hydrocarbons contained in solid bituminous minerals, in which the mineral sands undergo at least one digestion stage and subsequently one or more separations of solids and liquid layers, the latter being treated in turn and at least part recycled to the process. As known with the continuous increase of the petrol price the exploitation of the extensive deposits of tar sands or similar products, discovered in Canada, USA, Venezuela, Madagascar, etc., and consisting mainly of siliceous material impregnated with heavy and low API grade viscous hydrocarbons, has become an attractive alternative in the production of synthetic oil.
2. Description of the Prior Art
Many processes have been proposed for the treatment of tar sands such as: hot water processes, cold water and solvent processes, solvent only processes (see f.i. Canadian Pat. No. 611,587 U.S. Pat. No. 3,875,046 etc.)
The hot water processes, various problems notwithstanding, are currently used to extract bitumen from the Canadian sands of Athabasca, but fail when viscosity of the hydrocarbon to be recovered is too high or when the tar sands have, either originally or as a result of aging, a low interstitial water (known in technical jargon as "connate water") content.
Another disvantage of hot water processes is the production of a discharge stream (middlings) containing fines (generally &lt;44.mu.) and emulsioned hydrocarbon, which must be sent to tailing ponds outside the plant, thus presenting a serious economical and ecological problem and increasing energy consumption for the entire process.
Solvent processes, on the other hand, have always had the disadvantage of solvent losses so extensive as to render the process economically non viable. The phenomena of disaggregation of lumps of oil sand and of detaching the bitumen from the sand grains are dominated by two main parameters: the connate water content in the tar sands and the viscosity of the hydrocarbon to be recovered.
If the layer of connate water between the sand grains aggregate and the bitumen which mostly fills the cavity is sufficient, such connate water acts as lubricant during separation of the hydrocarbon.
In this case it is sufficient to operate above the hydrocarbon softening point (acting on the temperature, for example) to cause the separation, the hydrocarbon viscosity having a minimal influence on the speed (rate) of separation.
When, on the other hand, the layer of connate water is not sufficiently thick to provide adequate lubrication, the speed (rate) of hydrocarbon separation from the sand grains aggregate is almost entirely subject to the viscosity of the hydrocarbon itself.
This viscosity may be reduced, in these cases, by operating at a higher temperature and/or by adding as diluent a less viscous hydrocarbon, but in any case it is costly both in terms of energy and of diluent consumption.
In two recent Italian Patent Applications (No. 67236 A/80 corresponding to published European Patent Application 0034896 and No. 63438 A/80) Applicant and its affiliate RTR Rio Tinto have described a treatment for heterogeneous liquid materials and a complete process for the extraction of bituminous oils.